Scandium-Catalyzed Regio- and Stereoselective Cyclopolymerization

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Scandium-Catalyzed Regio- and Stereoselective Cyclopolymerization of Functionalized #,#-Dienes and Copolymerization with Ethylene Haobing Wang, Yanan Zhao, Masayoshi Nishiura, YANG YANG, Gen Luo, Yi Luo, and Zhaomin Hou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04275 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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Journal of the American Chemical Society

Scandium-Catalyzed Regio- and Stereoselective Cyclopolymerization of Functionalized ,-Dienes and Copolymerization with Ethylene Haobing Wang,†,II Yanan Zhao,§,II Masayoshi Nishiura,†,‡ Yang Yang,† Gen Luo,§ Yi Luo,*§ Zhaomin Hou*†,‡,§ †

Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 3510198, Japan ‡

Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan §

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China Supporting Information Placeholder ABSTRACT: The precise control of regio- and stereochemistry in the cyclopolymerization of heteroatom-functionalized α,ω-dienes is of much interest and importance, but has remained a challenge to date. We report herein the regio-, diastereoselective and stereoregular cyclopolymerization of ether- and thioether-functionalized 1,6-heptadienes by a half-sandwich scandium catalyst. The polymerization of 4-benzyloxy-1,6-heptadiene selectively afforded the corresponding benzyloxy-functionalized cyclic polymer composed of 1,2,4-cis-substituted-ethylenecyclopentane (ECP) microstructures in a isospecific fashion (95% mmm). In contrast, the polymerization of 4-phenylthio-1,6-heptadiene exclusively yielded 1,2-trans-1,4-cis-ECP units with high isotacticity (95% rrr). The DFT calculations revealed that an interaction between the scandium atom in the catalyst and the heteroatom in a diene monomer played an important role in controlling the regio- and stereochemistry of the diene cyclopolymerization. The copolymerization of functionalized 1,6-heptadienes with ethylene has also been achieved in a controlled fashion.

INTRODUCTION Organic polymers with cyclic repeating units in the polymer backbone are of much interest and importance in both academic research and industrial applications.1 Cyclic polyolefins bearing heteroatom functional groups are of special interest, as they may show improved beneficial properties compared to the heteroatom-free polymers. The cyclopolymerization of α,ω-dienes such as 1,5-hexadiene and 1,6-heptadiene by transition metal catalysts has demonstrated particular utility for the preparation of cyclopolyolefins.2-5 In this transformation, the precise control of the regio- and stereochemistry is highly important to obtain high-performance polymers. However, compared to the polymerization of unsubstituted α,ω-dienes, it is usually difficult to control the regio- and stereoselectivity of the cyclopolymerization of heteroatom-functionalized α,ωdienes, because of the lack of suitable catalysts that can not only show good compatibility with the heteroatom functional group but can also govern the stereochemistry of the heteroatom-substituent in addition to the regio- and stereoselectivity control of the common cyclization process. It has been previously reported that zirconocence catalysts could show compatibility with some sterically demanding siloxy-substituted α,ωdienes, such as 4-tert-butyldimethylsiloxy-1,6-heptadiene. However, the resulting siloxy-functionalized polymer products consisted of a mixture of trans- and cis-fused 1,3methylenecyclohexane (MCH) units without showing stere-

oselectivity at either the 1,3-dimethylene skeleton or the 5siloxy substituent (Scheme 1a, top).4 The cyclopolymerization of 4-tert-butyldimethylsiloxy-1,6-heptadiene by Fe and Co catalysts has been reported to yield polymers containing the five-membered ring 1,2- ethylene-4-siloxycyclopentane (ECP) Scheme 1. Transition-Metal-Catalyzed Cyclopolymerization of Heteroatom-Functionalized 1,6-Heptadienes

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units (Scheme 1a, bottom).5b Although 1,2-cis- or 1,2-trans diastereoselectivity could be achieved by choosing a Fe or Co catalyst, respectively, a control of the stereoselectivity on the 4-siloxy substituent was not achieved and no stereoregulary (tacticity) of the repeating ECP units was observed. Despite continued interest and recent advances in this area, the precise control of the regio- and stereochemistry in the cyclopolymerization of heteroatom-functionalized α,ω-dienes has remained a challenge to date.6 We have recently found that organo rare-earth metal catalysts can show unique activity and selectivity in various transformations of heteroatom-containing organic substrates,7,8 including the regio-, stereospecific polymerization of heteroatom-functionalized -olefins,8q,r because of the unique heteroatom-affinity of the rare-earth metal ions. These findings have promoted us to examine the cyclopolymerization of heteroatom-functionalized α,ω-dienes by rare-earth metal catalysts. We report herein the regio-, diastereoselective and stereoregular cyclopolymerization of heteroatom-functionalized 1,6heptadienes by a half-sandwich scandium catalyst, in which the regio- and stereoselectivity is uniquely controlled through the interaction between the catalyst metal center and the heteroatom. This protocol has efficiently afforded a new family of functionalized cyclic polyolefins with unprecedentedly high regio- and diastereoselectivity at all of the three chiral carbon centers together with highly stereoregular orientation of the cyclic ECP units (Scheme 1b). The cyclocopolymerization of the heteroatom-functionalized 1,6-heptadienes with ethylene is also described. RESULTS AND DISSCUSION

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Homopolymerization of Heteroatom-Functionalized 1,6Heptadienes by Half-Sandwich Scandium Catalysts. At first, we examined the polymerization of 4-benzyloxy-1,6-heptadiene (O-1) by using a combination of the C5H5-ligated scandium complex Sc-1 and [Ph3C][B(C6F5)4] as a catalyst.9 With the monomer to catalyst feed ratio [O-1]/[Sc-1] = 100/1, the polymerization slowly took place at room temperature, reaching 35% conversion in 24 h (Table 1, run 1). The resulting polymer contained a mixture of cyclized and uncyclized units, as shown by the NMR analyses. When the larger C5Me4Hligated scandium complex Sc-2 was employed, rapid polymerization was observed under the same conditions, selectively affording the cyclic polymer product with Mn = 28.8 ×103 g mol1 and Mw/Mn = 2.50 (Table 1, run 2). When the [O1]/[Sc-2] feed ratio was raised to 500/1, the monomer was quantitatively converted within 3 h, but an insoluble polymer product (1,2-dichlorobenzene at 145 °C) was yielded probably due to cross-linking (Table 1, run 3). The use of a further sterically demanding C5Me4SiMe3-ligated scandium complex Sc-3 with [O-1]/[Sc-1] = 100/1 yielded the cyclic polymer product with a narrower molecular distribution (Mn = 18.7 kg/mol and Mw/Mn = 1.50) (Table 1, run 4). As the [O-1]/[Sc-3] feed ratio was raised to 500/1, the number average molecular weight (Mn) of the resulting polymer showed a pronounced increase to 86.5 ×103 g mol1, and no cross-linked insoluble product was observed (Table 1, run 5). The polymerization of O-1 by Sc-3 proceeded even at -20 °C, affording the cyclic polymer product with higher stereoselectivity (Table 1, run 6). At 50 °C, the polymerization took place more rapidly but with slightly lower stereoselectivity (Table 1, run 7).

Table 1. Sc-Catalyzed Cyclopolymerization of Heteroatom-Functionalized 1,6-Heptadienesa

run

diene (D)

[Sc]

D/[Sc]

temp.

1 2 3 4 5 6 7 8 9 10 11 12

O-1 O-1 O-1 O-1 O-1 O-1 O-1 O-2 O-2 S-1 S-2 S-2

Sc-1 Sc-2 Sc-2 Sc-3 Sc-3 Sc-3 Sc-3 Sc-3 Sc-2 Sc-3 Sc-3 Sc-3

100/1 100/1 500/1g 100/1 500/1g 100/1 100/1 100/1 100/1 100/1 100/1 500/1g

20 20 20 20 20 -20 50 20 20 20 20 20

time (h) 24 0.5 3 1 48 12 0.5 24 96 48 3 3

conv. (%)b 35 100 100 100 70 84 99 64 30 57 100 98

Mnc (×103 g mol1) n.d.f 28.8 n.d.f 18.7 86.5 17.1 19.8 12.8 6.7 17.9 7.1 10.1

Mw/Mnc n.d f 2.50 n.d f 1.50 2.20 2.89 1.90 1.86 1.98 2.05 2.13 2.73

1,2-trans-1,4-cis/ 1,2,4-cisd n.d.f 6/94 n.d.f 6/94 6/94 2/98 12/88 9/91 11/89 75/25 94/6 94/6

tacticityd n.d.f 95% mmm n.d.f 95% mmm 95% mmm 98% mmm 90% mmm atactic 80% mmm n.d.f 95% rrr 95% rrr

Tg e (°C) n.d.f 10 n.d.f 11 11 11 13 50 47 13 28 31

Tme (°C) n.d.f 122 n.d.f 122 125 124 116 n.o.h 122 n.o.h 160 162

a Conditions: [Sc] (0.02 mmol), [Ph3C][B(C6F5)4] (0.02 mmol), diene (0.5 M), 20 °C. b Weight of polymer obtained/weight of monomer used. c Determined by gel permeation chromatography (GPC) in tetrahydrofuran at room temperature against polystyrene standard. d Determined by 13C NMR analysis. e Determined by differential scanning calorimetry (DSC). f n.d. = not determined. g diene (1.0 M). h n.o. = not observed.


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The 1H and 13C NMR analyses revealed that the cyclopolymer products (poly(O-1)) were exclusively composed of 1,2,4-cis-substituted ECP units (94%) (Figure 1a; see also Figures S9-S16). In the 13C{1H} NMR spectrum, the signal of the C1 atom in the ECP ring was identical to that of the C2 atom (δ 41.2), and the C3 signal was identical to that of the C5 atom (δ 37.5), suggesting that the C6 and C7 methylene substituents on the C1 and C2 atoms in the ECP ring should be cis to each other (Figure 1a). The benzyloxy-substituted carbon atom C4 showed a sharp signal at δ 80.0, indicating that the configuration of the benzyloxy substituent was also well fixed. A well-expanded NOESY spectrum showed distinct correlation between the C4H proton (3.98 ppm) and the C1H as well as C2H proton (1.78 ppm) (Figure S16), suggesting that the benzyloxy group on C4 should be stereoselectively oriented cis to the CH2 substituents on the C1 and C2 atoms in the ECP ring. These results are indicative of a highly regulated 1,2,4-cis diastereoselective ECP configuration. More remarkably, the 13 C{1H} NMR signals of all the carbon atoms in the ECP units appeared as well-resolved singlets, suggesting that the ECP units were oriented with high stereoregularity (mmm = 95%, isotactic).10 The DSC analysis of the polymer products revealed melting temperatures (Tm) at 122-125 °C (Table 1, runs 2-7; Figures S18, S20, S22), in agreement with the high stereoregularity of the ECP units. The cyclopolymerization of 4-phenoxy-1,6-heptadiene (O2) by Sc-3 also took place in a regio- and diastereoselective fashion, selectively affording the corresponding phenoxysubstituted poly(ECP) in a 1,2,4-cis fashion (91%), but the reaction was much slower than that of the benzyloxy analog O1 under the same conditions (Table 1, run 8). In contrast with the high tacticity of poly(O-1), poly(O-2) synthesized by Sc-3 showed poor stereoregularity (atactic) (Figure S32). These results suggest that the substituent on the oxygen atom in a ether-substituted 1,6-heptadiene could show significant influences on the activity and stereoregularity of the cyclopolymerization of the diene unit.8q The use of the smaller C5Me4Hligated Sc-2 catalyst improved the tacticity (80% mmm), but the reaction remained sluggish (30% conversion in 96 h) (Table 1, run 9, Figure S38). It is also worth noting that the cyclopolymerization of the oxygen-free 1,6-heptadiene by Sc-3 yielded the six-membered ring MCH units (91%) with no stereoselectivity,3f standing in sharp contrast with the exclusive formation of the five-membered ring ECP units in the polymerization of the ether-functionalized 1,6-heptadienes O1 and O-2. These results suggest that the ether group may show critical influences not only on the stereoselectivty but also on the regioselectivity of the diene cyclopolymerization. To further probe the heteroatom effects, we then examined sulfur-functionalized 1,6-heptadienes by using catalyst Sc-3. Compared to O-1, the sulfur analog 4-benzylthio- 1,6heptadiene S-1 was much less active and less selective under the same conditions, which afforded ploy(S-1) composed of a 75:25 mixture of the 1,2-trans- and 1,2-cis- ECP units with 57% conversion in 48 h (Table 1, run 10; Figures S45-S46). By contrast, 4-phenylthio-1,6-heptadiene (S-2) showed muchhigher activity and higher stereoselectivity than S-1, selectively yielding the thioether-functionalized cyclic polymer consisting of isotactic 1,2-trans, 1,4-cis-ECP units with rrr = 95% (Table 1, runs 11 and 12; Figure 1b).10,11 In the 13C{1H} NMR

Figure 1. 13C{1H} NMR spectra (125 MHz, 1,1,2,2-C2D2Cl4, 26.8 °C). (a) Poly(O-1) (Table 1, run 4), (b) Poly(S-2) (Table 1, run 11).

spectrum of poly(S-2), the chemical shifts of the C1 (δ 44.9) and C2 (δ 46.5) carbon atoms in the ECP ring were significantly different, and so were those of the C3 (δ 39.9) and C5 (δ 40.7) atoms, suggesting that the C6 and C7 methylene substituents on the C1 and C2 atoms should be trans to each other (Figure 1b). In the HETCOR spectrum, the difference in chemical shift between the two protons on the C5 atom (δ 40.7, 1.2 and δ 40.7, 2.4) was much bigger that on the C3 atom (δ 39.9, 1.7 and δ 39.9, 1.9) (Figure S53), probaly because of the influence of the PhS substituent on the C4 atom. This suggests that the PhS substituent on the C4 should be diastereoselectively oriented cis to the C1 and trans to the C2 substituents on the ECP ring (i.e., 1,2-trans, 1,4-cis configuration). The 13 C{1H} NMR signals of all the carbon atoms in the ECP units appeared as well-resolved singlets, suggesting that the ECP units should be oriented in a highly stereospecific fashion (rrr = 95%, isotactic). In agreement with the high stereoregularity, ploy(S-2) showed high melting temperatures (Tm) at 160162 °C and apparent crystallization temperature at 83-86 °C (Table 1, runs 11 and 12; Figures S59, S61). The selective formation of the 1,2-trans, 1,4-cis-ECP units in the polymerization of S-2 stands in sharp contrast with the exclusive formation of the 1,2,4-cis ECP configurations in the case of O-1 and O-2, again demonstrating the significant influence of the heteroatom in the present polymerization. DFT Calculations of the Cyclopolymerization of S-2 and O-1. To acquire insights into the high regio- and stereoselectivity observed above, we preformed the DFT calculations of the cyclopolymerization of S-2 and O-1 catalyzed by the cationic species (Cat) generated by the reaction of Sc-3 with 1 equivalent of [Ph3C][B(C6F5)4]. In the case of S-2, once the sulfur atom and a vinyl group in the diene are coordinated to the Sc center, the S-bonded prochiral C4 atom in S-2 becomes chiral because of desymmetrization. Two coordination configurations (i.e., AS(4R) and AS(4S)) of C4 were therefore considered for the insertion/cyclization reactions, respectively (Figure 2). In both pathways, the si-coordination insertion mode was calculated to be more kinetically favorable than the re-fashion (19.8 and 21.6 vs 29.1 and 28.9 kcal/mol, Figure 2), which is consistent with the previous DFT analyses of the Sc-catalyzed polymerization of 4-phenylthio-1-butene.8q It has also been found that si-AS(4R) is more stable than si-AS(4S) by 2.8



kcal/mol and the formation of si-BS(1S,4R) is more favored both kinetically and thermodynamically in comparison with siBS(1S,4S) (barriers of 19.8 vs 21.6 kcal/mol and reaction energies of −0.3 vs 2.8 kcal/mol, Figure 2). Once the more stable si-BS(1S,4R) was generated, different coordination/insertion fashions (re- or si-) of the remaining C=C bond could yield two diastereo-cyclic products with different configurations. The intramolecular coordination of the remaining C=C bond to the Sc atom in si-BS(1S,4R) could yield si-CS(1S,4R) or reCS(1S,4R). Although si-CS(1S,4R) is 0.8 kcal/mol higher in energy than re-CS(1S,4R), the subsequent insertion of the C=C double into the Sc−alkyl bond in si-CS(1S,4R) via si-TS-

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CS(1S,2S,4R) (ΔG‡ = 12.6 kcal/mol) to give si-DS(1S,2S,4R) (si-insertion, 1,2-trans-1,4-cis product) is more favorable than the pathway of re-CS(1S, 4R)  re-TS-CS(1S,2R,4R)  reDS(1S,2R,4R) (re-insertion, energy barrier of 26.8 kcal/mol, 1,2-cis-1,4-trans product). A further calculation of the reaction of the second S-2 monomer with si-DS(1S,2S,4R) also indicated more favorable si-insertion and subsequent si-cyclization process to give a second 1,2-trans-1,4-cis ECP unit (Figure S118). Repetition of these si-insertion and si-cyclization modes would therefore afford the isotactic polymer product, being in agreement with the experimental observation.

Figure 2. Computational analysis of the insertion/cyclization of first molecule of S-2 by the cationic species (Cat) generated by the reaction of Sc-3 with [Ph3C][B(C6F5)4]. (A) DFT-calculated energy profile. (B) Structures of the stationary points shown in the energy profile. The less favored pathway is pale-colored. R = CH2C6H4NMe2-o, TS: transition state, Gsol: Relative Gibbs free energy in solution.


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Figure 3. Computational analysis of the insertion/cyclization of first molecule of O-1 by the cationic species (Cat) generated by the reaction of Sc-3 with [Ph3C][B(C6F5)4]. (A) DFT calculated energy profile. (B) Structures of the stationary points shown in the energy profile. The less favored pathway is pale-colored. R = CH2C6H4NMe2-o, TS: transition state, Gsol: Relative Gibbs free energy in solution.

In the case of O-1 (Figure 3), the si-coordination insertion mode of the first C=C bond is also more kinetically favorable than the re-fashion. In contrast to S-2, however, in the two sicoordination insertion modes, the pathway of si-AO(4S)siTS-AO(1S,4S)si-BO(1S,4S) is more energetically favored than that of si-AO(4R)si-TS-AO(1S,4R)si-BO(1S,4R) (Figure 3). This indicates that the S configuration on the C4 atom of O-1 becomes more favorable during the si-coordination insertion of the first C=C bond. In the cyclization step, the reCO(1S,4S)-involved intramolecular insertion of the remaining C=C bond into the Sc‒alkyl bond is significantly more favorable than that of si-CO(1S,4S) (barriers of 9.7 + 5.8 = 15.5 vs 22.9 + 5.8 = 28.7 kcal/mol, Figure 3). The incorporation of the second O-1 monomer was also calculated to favor the similar way as the first monomer adopted (Figure S119). Repetition of these si-insertion and re-cyclization modes could therefore afford the isotactic polymer product, being in line with the experimentally observed selectivity.

Mechanism of the Sc-Catalyzed Cyclopolymerization of S-2 and O-1. On the basis of the DFT calculations described above, the possible scenarios of the regio- and stereospecific cyclopolymerization of S-2 and O-1 by Sc-3 are shown in Schemes 2a and 2b, respectively. The chelation of the Sc atom by S-2 with the S atom and a C=C bond occurs in a si-coordination fashion to give the most favorable si-AS(4R) configuration (Scheme 2a). The subsequent 2,1-insertion of the Sc-coordinated C=C bond into the Sc−CH2C6H4NMe2-o bond would generate the siBS(1S,4R) intermediate. The intramolecular si-coordination of the remaining C=C bond to the Sc atom followed by 1,2sinsertion into the Sc−C σ bond via si-CS(1S,4R) may then take place to give the thermodynamically and kinetically favored 1,2-trans cyclopentane species si-DS(1S,2S,4R). Similarly, the si-coordination of the second monomer S-2 to the Sc atom (to give si-ES(4R)) and the subsequent si-2,1-insertion would yield si-FS(1S,4R). The intramolecular si-1,2-insertion (cyclization) of the remaining C=C bond via si-GS(1S,4R) should generate another ECP unit (see si-HS(1S,2S,4R)) with a configuration



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Scheme 2. Possible Mechanisms of Regio- and Stereoselective Cyclopolymerization of S-2 (a) and O-1 (b) by Sc-3

identical to that in si-DS(1S,2S,4R). The incorporation of the third S-2 monomer into si-HS(1S,2S,4R) in the same manner should then afford the 1,2-trans, 1,4-cis-fused ECP microstructures with isotactic stereoregularity (rrr-IS). In the case of monomer O-1 (Scheme 2b), the chelation of the Sc atom with the O atom and a C=C bond would preferably give the si-AO(4S) configuration, as shown by the DFT analysis. The subsequent 2,1-insertion of the Sc-coordinated C=C bond into the Sc−CH2C6H4NMe2-o bond should yield

the si-BO(1S,4S) intermediate. The intramolecular recoordination of the remaining C=C bond to the Sc atom followed by 1,2-sinsertion into the Sc−C σ bond via re-CO(1S,4S) could give the thermodynamically and kinetically favored 1,2,4-cis-subsititued ECP species re-DO(1S,2R,4S). Similarly, the successive incorporation of the second and third O-1 monomers in a si-2,1-/re-1,2-insertion/cyclization manner would afford the isospecific 1,2,4-cis-fused ECP units mmm-IO.


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Table 2. Copolymerization of Heteroatom-Functionalized 1,6-Heptadienes with Ethylene by Sc-3a

run 1 2 3 4 5 6 7 8

diene (D) O-1 O-2 O-2 O-2 S-1 S-2 S-2 S-2

D/[Sc-3] 100/1 100/1 500/1 1000/1 100/1 100/1 200/1 500/1

time (min) 10 5 30 45 90 0.5 1.5 3

yield (g)

diene conv. (%)

0.30 0.47 2.16 3.56 0.40 0.40 0.79 2.00

74 50 62 65 88 62 77 94

activity (g molSc1 h1 atm1) 282 216 153 13.3 2400 1580 2000

Mnb (×103 g mol1) 14.1 55.9 100.8 147.9 24.4 29.6 18.5 14.7

Mw/Mnb 1.53 2.05 2.20 1.74 1.88 1.45 2.35 3.89

i.r.c (%) 100 9 15 25 74 19 35 77

1,2-trans-1,4-cis/ 1,2,4-cisd 6/94 7/93 7/93 7/93 75/25 94/6 94/6 94/6

Tg e (°C) 10 -19 -17 -9 8 14 21 29

Tme (°C) 116 126 127 n.o.f n.o.f 120 113/139 n.o.f

a Conditions: [Sc-3] (0.02 mmol), [Ph3C][B(C6F5)4] (0.02 mmol), ethylene (1 atm), toluene (50 mL) at 20 °C. b Determined by GPC in 1,2-dichlorobenzene at 145 °C against polystyrene standard. c Incorporation ratio of D, determined by 1H NMR analysis. d Determined by 13C NMR analysis. e Determined by DSC. f n.o. = not observed.

Copolymerization of Heteroatom-Functionalized 1,6Heptadienes with Ethylene. With the establishment of the regioand stereoselective cyclopolymerization of the heteroatomfunctionalized 1,6-heptadienes, their copolymerization with ethylene was also examined (Table 2). Under the coexistence of O-1 and ethylene (1 atm) with [O-1]/[Sc-3] = 100/1, only the homopolymer of O-1 was formed (Table 2, run 1). This is probably due to the strongly coordinating ability of the benzyloxy group in O-1, which may hamper the coordination/insertion of ethylene at the catalyst metal center. By contrast, the copolymerization of the phenoxy-substituted monomer O-2 with ethylene occurred rapidly under the same conditions, although the homopolymerization of O-2 was less efficient. With [O-2]/[Sc-3] = 100/1 under 1 atm of ethylene at room temperature, a copolymer product with Mn = 55.9 ×103 g mol1 and O-2 incorporation of 9 mol% was obtained in 5 min (Table 2, run 2). As the [O-2]/[Sc-3] ratio was raised to 500/1 and 1000/1, the molecular weight (Mn) of the resulting copolymers significantly increased to 100.8 × 103 g mol1 and 147.9 ×103 g mol1, and the O-2 incorporation ratio increased to 15 mol% and 25 mol%, respectively (Table 2, runs 3 and 4). The 13 C{1H} NMR analyses revealed that the copolymers contained predominantly isolated 1,2,4-cis ECP units (Figures S74-S81). Uncyclized pendency vinyl group was not observed, suggesting that the cyclization step competed well against ethylene insertion in the present copolymerization. The copolymerization of S-1 and ethylene (1 atm) with [S1]/[Sc-3] = 100/1 afforded the corresponding sulfurfunctionalized copolymer with 74 mol% incorporation of the S1 monomer and a 75/25 mixture of the 1,2-trans-1,4-cis- and 1,2,4-cis-ECP units (Table 2, run 5). The copolymerization of S-2 and ethylene with [S-2]/[Sc-3] = 100/1, 200/1, and 500/1 gave the copolymers with the S-2 incorporation ratio = 19, 35, and 77 mol%, respectively (Table 2, runs 6-8). The copolymers possessed predominantly the 1,2-trans, 1,4-cis-ECP units, similar to the homopolymerization of S-2. Transformations of a Cyclic Polymer. Replacement of the benzyl groups in poly(O-1) (Mn = 86.5 ×103 g mol1, Table 1, run 4) with the triethylsilyl group could be achieved by reaction with Et3SiH in the presence of a catalytic amount of B(C6F5)3, quantitatively affording the Et3SiO-substituted

poly(OSiEt3) (Scheme 3). Poly(OSiEt3) exhibited a much lower Tg (–10 °C) than that of poly(O-1) (Tg = 11 °C) without showing a Tm or Tc in the DSC profile (Figure S111). The protonolysis of poly(OSiEt3) by aqueous HCl in THF gave the corresponding HO-functionalized polymer poly(OH). Poly(OH) demonstrated excellent thermal stability (Tg = 105 °C, Tm = 261 °C, Td (decomposition temperature) = 400 °C; Figures S114 and S115) and excellent solvent resistance (insoluble in dichlorobenzene at 145 ºC). Scheme 3. Transformations of Poly(O-1)

CONCLUSION In summary, we have achieved for the first time the regio-, diastereoselective and highly stereoregular cyclopolymerization of ether- and thioether-functionalized 1,6-heptadienes and their copolymerization with ethylene by using a halfsandwich scandium catalyst such as Sc-3. The unprecedented diastereoselectivty and stereoregularity could be ascribed to the unique interaction between the catalyst metal center and the heteroatom in a diene monomer, as shown by DFT calculations. This protocol has afforded a new family of heteroatomfunctionalized cyclic polyolefins which were difficult to prepare previously. Our findings may also help design and synthesize new catalysts for efficient and regio-, stereoselective cyclopolymerization and related transformations of various functional dienes.

EXPERIMENTAL SECTION Materials and methods. All manipulations of air and moisturesensitive compounds were performed under a dry nitrogen atmosphere by use of standard Schlenk techniques or a nitrogen-filled Mbraun glovebox. Nitrogen was purified by being passed through a Dryclean column DC-A4 (4 Å molecular sieves, Nikka Seiko Co.) and a Gasclean GC-XR column (Nikka Seiko Co.). Solvents were purified by an Mbraun SPS-800 Solvent Purification System and dried over fresh Na chips in the glovebox. [Ph 3C][B(C6F5)4] (97%) was purchased from Strem Chemical Corporation and used without purification. Rare-earth catalysts Cp′Sc(CH2C6H4NMe2-



o)2 (Sc-1: Cp′ = C5H5, Sc-2: Cp′ = C5Me4H, and Sc-3: C5Me4SiMe3), were synthesized according to the literatures.9,12 4Functionalized-1,6-heptadienes (O-1, O-2, S-1 and S-2) were synthesized from 4-hydroxy-1,6-heptadien (purchased from Aldrich) according to the literatures.8q,13 4-Functionalized-1,6-heptadienes (O-1, O-2, S-1, and S-2) were purified by distillation from Al(octyl)3 (25 wt.% in hexanes) before use. The deuterated solvents benzene-d6 (99.6 atom % D), CDCl3 (99.8 atom % D), and 1,1,2,2-tetrachloroethane-d2 (99.6 atom % D) were obtained from Kanto Chemical Co. Inc. and Cambridge Isotope. The NMR data of the polymers were obtained on a Bruker AVANCE III HD 500 NMR (FT, 500 MHz for 1H; 125 MHz for 13C) spectrometer with CDCl3 (at 26.8 °C) or 1,1,2,2C2D2Cl4 (at 120 °C) as a solvent. The chemical shifts for 1H NMR were recorded in ppm downfield from tetramethylsilane (TMS) with the solvent resonance as the internal standard (7.16 ppm for C6D6, 7.26 ppm for CDCl3, 6.0 ppm for 1,1,2,2-C2D2Cl4). The chemical shifts for 13C{1H} NMR were recorded in ppm downfield using the central peak of C6D6 (128.06 ppm), CDCl3 (77.16 ppm), 1,1,2,2-C2D2Cl4 (73.78 ppm) as the internal standard. Coupling constants (J) are reported in Hz and refer to apparent peak multiplications. The abbreviations s, d, t, q and m stand for singlet, doublet, triplet, quartet and multiplet in that order. The molecular weights and the molecular weight distributions of homopolymers were determined at 40 °C by gel permeation chromatography (GPC) on a HLC-8320 GPC apparatus (Tosoh Corporation). THF was employed as an eluent at a flow rate of 0.35 mL/min. The molecular weights and the molecular weight distributions of all the copolymers were determined at 145 °C by high temperature gel permeation chromatography (HT-GPC) on a HLC-8321GPC/HT apparatus (Tosoh Corporation). 1,2Dichlorobenzene (DCB) was employed as an eluent at a flow rate of 1.0 mL/min. The calibration was made by use of polystyrene standard (Tosoh Corporation). The DSC measurements were performed on a DSC 7000X (HITACHI) at a rate of 10 °C/min. Any thermal history difference in the polymers was eliminated by first heating the specimen to 180 °C (or 300 °C), cooling at 10 °C/min to −100 °C (or −30 °C) and then recording the second DSC scan. TGA was recorded on TGA-50 thermogravimetric analyzer (Shimadzu). The temperature range is 30 to 500 °C at the heating rate of 10 °C min-1. General procedure for the homopolymerization of functionalized dienes. Polymerization of O-1 (Table 1, run 3): In a THF-free glovebox, a toluene solution (2 mL) of [Ph3C][B(C6F5)4] (18.4 mg, 0.02 mmol) was slowly added to a toluene solution (2 mL) of (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2 (Sc-3, 10.1 mg, 0.02 mmol) under magnetic stirring in a 10 mL glass tube with cap. After the mixture was stirred for 5 min, O-1 (404.0 mg, 2.0 mmol) was added into the reaction solution. The tube was sealed and kept stirring in the glovebox at room temperature for 1 hour. The polymerization was quenched by addition of methanol (10 mL). Then the mixture was poured into methanol (50 mL) to precipitate the polymer product. The precipitated polymer was dried under vacuum at 60 °C to a constant weight (404 mg, 100% yield). General procedure for the copolymerization of functionalized dienes with ethylene (Table 2, run 2). In a THF-free glovebox, a toluene solution (45 mL) of O-2 (376 mg, 2 mmol) was charged into a three-necked flask with a magnetic stir bar. The flask was taken outside, set in a water bath (20 °C), and connected to a wellpurged Schlenk ethylene line and a paraffin oil-sealed stopper by use of a three-way cock. Ethylene (1 atm) was introduced into the system and was saturated in the solution by stirring for 2 min. A toluene solution (5 mL) of (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2 (Sc-3) (10.1 mg, 0.02 mmol) and [Ph3C][B(C6F5)4] (18.4 mg,

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0.02 mmol) was then added through a syringe under vigorous stirring. The polymerization was quenched after 5 minutes by adding methanol (50 mL). The polymer product was collected by filtration, washed with methanol, and then dried in vacuum at 100 °C to a constant weight (0.47 g). The resulting polymer was soluble in hot toluene, dichlorobenzene and 1,1,2,2tetrachloroethane. Solvent fractionation experiments suggest that no homopolyethylene was formed. The phenoxy substituted cyclic unit content in the copolymer was calculated from the 1H NMR analysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.xxxxxx. Experimental and computational details, NMR spectra and other characterization data, including Figures S1-S119 (PDF).

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Author Contributions II H.W and Y.Z contributed equally to this work. ORCID Haobing Wang: 0000-0002-9104-1726 Yanan Zhao: 0000-0002-3928-3429 Masayoshi Nishiura: 0000-0003-2748-9814 Yang Yang: 0000-0002-3041-8593 Gen Luo: 0000-0002-5297-6756 Yi Luo: 0000-0001-6390-8639 Zhaomin Hou: 0000-0003-2841-5120 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported in part by A Grant-in-Aid for Scientific Research (A) (JP19H00897), a Grant-in-Aid for Scientific Research (S) (26220802), a Grant-in-Aid for Scientific Research (C) (No. 17K05824) a Grant-in-Aid for Scientific Research on Innovative Areas (17H06451) from JSPS and by the ImPACT Program of Council for Science, Technology and Innovation, the Cabinet Office, Government of Japan and the National Natural Science Foundation of China (nos. U1862115, 21674014). The authors also thank the RICC (RIKEN Integrated Cluster of Clusters) and the Network and Information Center of Dalian University of Technology for computational resources.

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Scandium-Catalyzed Regio- and Stereoselective Cyclopolymerization

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